This invention relates to fluid sampling and has particular relationship to the sampling of gases, predominately air, for selected components. Typically, the fluid sampling with which this invention concerns itself is the sampling of air in plants for harmful components pursuant to the Occupational Safety and Health Act (OSHA) or the sampling of the air at a site such as Love Canal contaminated by chemical waste. Sampling of this type is typically carried out by a sampler carried or "worn by" the person subjected to the harmful components in the air. The sampling may be carried out throughout the working day of the personnel involved or during measured intervals. On occasion, it is also desireable that the flow of air through the sampler be limited.
A sampler includes a pump which pumps the air through a channel including the collecting or processing device which collects and/or measures the concentration of the harmful components. The processing device is usually interposed in the inlet (suction side) to the pump; it may also be in the outlet (pressure side) particularly in the case of bag collection of chemicals. The components sampled are gases or vapors or solid particulate. The air sampled contains dirt such as dust and other debris and it is necessary that such dirt be filtered out prior to entering the pump. The pump is protected by a so-called pump filter through which the pumped air flows as it is drawn into the channel. The function of the pump filter is to protect the pump from this dirt and debris. The filter becomes loaded with dirt during operation and presents an increasing pressure increment to the pump. In addition, there is an increasing impedance to the flow of pumped air through the channel and the flow or the time rate of the flow of air decreases unless the effect of the filter on the flow is compensated. It is necessary that the filter be replaced timely to preclude improper operation. The flow may also vary for other reasons than accumulation of dirt on the filter. In accordance with the teachings of the prior art, a differential pressure switch is interposed in the outlet of the flow channel to suppress the variation in the air flow. This pressure switch responds to the differential between the pressure in a chamber and the pressure outside of the chamber. The chamber is connected to the region outside through a needle valve. The pressure switch opens and closes repeatedly responsive to the pulsations produced by the pumping. One disadvantage of this expedient is that the repeated operation of the pressure switch has a tendency to cause fatigue in its linkages. Another, and more important, disadvantage is that traces of contaminents such as dirt clog or impede the operation of the needle valve precluding satisfactory, stable and reliable response of the switch to pressure variations in the flow channel. Another drawback is that additional pumping power is required to maintain the pressure required for sensing because an increment of pressure is absorbed for maintaining the control power.
It is an object of this invention to overcome the disadvantages of the prior art and to provide a fluid sampler which shall not include moving parts subject to fatigue and which shall operate reliably and with stability to maintain the flow of fluid being sampled substantially constant.
In the practice of the sampling art, it is desireable to vary selectably the flow; i.e., the number of milliliters per second of fluid, which flow through the sampler, specifically through the collecting or processing device. It is an object of this invention to provide a sampler which, (A) shall have the facility of sampling fluid flowing through the collecting device at selectably preset precise rates and (B) shall maintain the preset flow over a wide range of back pressure.
In prior-art practice, the pump filter was not readily accessible or visible. During the use of such a sampler, the pump filter would ultimately become clogged materially deteriorating the operation of the sampler and its reliability. In addition, the pump filter would cease to perform its filtering function and dirt would accumulate in the pump and handicap the operation of the valves. It would then become necessary to dismantle the sampler completely, clean the pump and replace the pump filter. It is an object of this invention to overcome this disadvantage of the prior art and to provide a sampler in which the state of the pump filter shall be readily observable and the filter shall be readily accessible for replacement. It is also an object of this invention to provide a sampler in which the entraining of moisture by the sampled fluid shall be precluded.